The front and rear windows of cars or the windows of other motor vehicles that are not opened and closed are frequently glazed by the method known as direct glazing. Direct glazing is a technique by which glass sheets are directly mounted in position on the body of a vehicle with the aid of a sealant having high adhesive strength. The sealant for use in the direct glazing of cars is required to have the following characteristics.
1. The sealant should have sufficiently high mechanical strength to resist breakage; specifically it should have a tension at break of not less than 3 MPa.
2. The sealant should provide high glass-sealant adhesive strength and high vehicle-body-sealant adhesive strength.
3. The sealant should have a fair degree of rigidity and yet sufficient rubber elasticity to prevent propagation of vibrations and shocks to the glass. Specifically, it should be a rubber elastomer having a modulus at 100% elongation of not less than 1 MPa and an elongation at break of not less than 200%.
4. For expediting the glazing work, the sealant should have the property to cure quickly after installation of the glass member on the vehicle body; specifically a complete cure should be obtained within 10-40 minutes, preferably 10-20 minutes, in air at room temperature.
The sealant used in direct glazing today is a moisture-setting urethane sealant.
The present invention has its object to provide a method for direct glazing with use of a novel sealant in lieu of said urethane sealant. More specifically, the object of the invention is to provide a method for direct glazing with use of a novel curable composition insuring sufficient mechanical strength, adhesive strength, rubber elasticity, and workability.
The inventors of the present invention found that a sealant comprising an oxyalkylene polymer having a reactive silicon (Si) group containing a hydroxyl or hydrolyzable group bound to a silicon atom and capable of crosslinking by silanol condensation reaction has the above-mentioned properties and is suitable as a sealant for direct glazing and that it can be provided as a one-part system that is easy to work with. It was also found that although an oxyalkylene polymer having such a reactive Si group gives a cured product with somewhat insufficient tension at break, addition of carbon black as a filler results in a marked improvement in tension at break. In addition, it was found that while the cured product of an oxyalkylene polymer containing said reactive Si group exhibits rubber elasticity, a plasticizer is preferably added for further enhancement of rubber elasticity. Although the reason is unknown, when a plasticizer of low molecular weight, such as 2-ethylhexyl phthalate (DOP), is added and the resulting sealant is stored for many months, the setting rate is decreased to adversely affect workability. However, it was discovered that when an oxyalkylene polymer that is a plasticizer of high molecular weight is used and the resulting sealant is stored, no decrease in setting rate is encountered.